We know that there is air pressure on top of us
all the time.But is the same amount of pressure
on us inside a building as outside?

Question Date: 2005-05-06

Answer 1:

That's a good question! I'd never thought of it
before. Because most buildings are not built
air-tight, air pressure can equalize inside the
building, so yes, you are generally under the same
air pressure inside a building as you are outside.
Pressure works horizontally as well as vertically,
and air can be forced into or out of a building
through cracks in windows, ventilation systems,
open doors and windows, etc.

The difference
in air pressure inside and outside a building due
to the difference in the weight of the air is
probably small compared to the difference in air
pressure inside and outside a building due to
other effects. As wind blows past a building, it
will cause a large difference in air pressure
inside versus outside the building, as will
differences in air temperature (if the building is
heated or cooled, for example) and humidity.
According to the EPA, even an elevator moving
inside a building will create air pressure
differences great enough to generate air flow
inside the building. The EPA and other regulatory
agencies, as well as architects and building
contractors, study the flow air through large
buildings for health (ventilation) and comfort
reasons.

I guess the real question is: If
you could somehow make a building completely
airtight and never open any doors or windows and
never cause any changes in air flow inside the
building (no fans, elevators, etc), would you be
able to measure a small difference in air pressure
inside the building versus outside the building?

Answer 2:

Actually, it is perhaps not correct to describe
the pressure as being "on top of us" because the
pressure is everywhere. It is within us as well.
That is why we don't feel it. The technical term
for such a pressure is "isotropic" (meaning it is
the same from all directions) or "hydrostatic"
meaning that there is a pressure-transferring
medium (air in this case) that allows the pressure
to be the same everywhere: inside us, in a car, in
buildings ...

The way this pressure would
change is if we move further from the center of
the earth, because the pressure is caused by the
gravitational attraction of the earths atmosphere
to the earth. So in Mexico City (which is at 8000
feet) the pressure is less than in Los Angeles
(which is at sea level). It could also change if
we either remove the pressure transfer medium (in
a vacuum) or we fill lots of it in a small volume
(a car tire).

Answer 3:

The air pressure is all around us, no matter if
you are inside or outside of a building. In order
to change the pressure, you have to change the
amount of molecules of air around you. This is
possible either using a pressure chamber or moving
upwards or downwards from the sea level. If
you've ever been to the top of a tall mountain,
you may have noticed that your ears pop and you
need to breathe more often than when you're at sea
level. As the number of molecules of air around
you decreases, the air pressure decreases. This
causes your ears to pop in order to balance the
pressure between the outside and inside of your ear.

Answer 4:

Usually there is the same pressure inside a
building as outside because there are openings
such as vents and windows which equalize the
pressure. One way to tell if the pressure is not
the same on two sides of a door is if a door is
difficult to open for "no reason." This usually
means that the room you are trying to push the
door open towards has higher air pressure.

Answer 5:

The same air pressure is on us inside a building
as outside - as long as the building has air
connections to the outside (which it normally
does). If you were in an airtight room and extra
air was pumped in or out, the pressure would
change -- that is the only time the inside of a
building is different than the outside

Answer 6:

Generally, yes. Sometimes, though, the pressure
might be kept a little higher inside a building by
using the equipment that circulates air. The
pressure difference would be rather small but you
can tell if the pressure is higher or lower inside
a building by noticing if there is a breeze when
you open the door (notice direction of the breeze
as well). Why do you think you might like to have
the pressure inside a building a little higher
than outside?